Comminuted cellulosic fibrous material, such as slurried wood chips, may be refined in one or more refiners for producing pulp for use in fiberboard and the like. Process steam is inherently generated during the refining process, forming a mixture of mechanical pulp and process steam. In addition, it is sometimes desirable to add resin to the mixture. Therefore, some refining systems include feed lines for adding resin. After comminution and the addition of resin, the mixture is generally dried in a fiber dryer, such as a flash tube fiber dryer.
During the manufacture of the pulp, gaseous volatile organic compounds (VOCs) are generated and emitted. Emissions from fiber dryers contain relatively high levels of VOCs, which may be above acceptable emission levels pursuant to Federal Maximum Achievable Control Technology (MACT) regulations. In addition, VOC levels may be high if resin is added after refining because many resins, such as urea formaldehyde based resins, release VOCs and other impurities after the refining process. The prevalent control technology for reducing emissions to VOC compliance levels is a regenerative thermal oxidizer (RTO). However, RTOs typically have high capital costs and operating costs due to the relatively large volume of dryer exhaust that must be treated.
In an attempt to reduce dryer exhaust emissions, some refiner systems separate the steam from the fiber before the fiber enters the dryer. It is well known that the steam carrying the wood fiber from the refiner to the dryer contains a relatively large percentage of the VOC emission components. Thus, various attempts have been made to provide an efficient system employing steam separation for reducing VOC emissions.
A cyclone is a common means of separating a solid material being conveyed by a gas. Some refining systems use a pressurized cyclone for separating the fiber from the steam. The separated steam generated in the cyclone may be condensed, cleaned using scrubbers, or processed using some other means known in the art. The fiber is then transported to a dryer. Ideally, a relatively high percentage of the “dirty” steam (i.e. steam containing VOCs and other impure emission components), for example 75% or more, would be removed from the fiber. However, the current separators used in conventional systems do not attain such levels of separation.
Furthermore, many pressurized cyclones and some pressurized separators use a percentage of the steam from the refiner to move the fiber to the dryer. Thus, a sufficient amount of dirty steam is required to carry the fiber to the dryer. This limits efficiency, given a relatively large portion of dirty steam is generally required to transport the fiber to the dryer.
In an attempt to reduce the percentage of dirty steam used for transporting the fiber, some systems add additional “clean” steam to the fiber prior to steam separation. Although emissions may be slightly reduced, such systems are inefficient because excessive quantities of clean steam must be provided. Furthermore, such systems may still fail to achieve acceptable VOC emission levels.
Other systems use a non-pressurized cyclone for steam separation. A higher percentage of steam separation is typically achieved compared to pressurized systems. Non-pressurized systems are more effective at separating the steam, because at ambient pressures the steam has maximum volume and less steam will be carried out of the cyclone in voids between the fibers. Also, more of the water and VOCs will be in vapor form at lower pressures. Such conventional systems typically provide that the fiber is mixed with the resin prior to steam separation. The mixture then undergoes steam separation, after which the fiber empties directly from the separator into the dryer. Although non-pressurized systems are effective at separating steam, such systems typically fail to achieve adequate blending between the resin and fiber. Furthermore, fiber clumping, wherein the fiber lumps or balls, is prevalent in such systems, particular when the fiber exits from the cyclone directly into the dryer. Furthermore, such systems often cause resin spotting on the fibers due to inadequate dispersal of the mixture upon entering the dryer.
Additional problems and/or concerns must be addressed when resin is added to the fiber/steam mixture. Some resins, such as urea formaldehyde based resins, are typically added to the fiber/steam mixture prior to steam separation because such resins release VOCs, such as formaldehyde, during processing. In this way, VOCs emitted may be separated and processed along with the dirty steam. However, the addition of resin to the mixture upstream of the cyclone tends to clog the cyclone. Resin build-up must be periodically removed from the cyclone. This increases manufacturing cost.
In an attempt to eliminate problems associated with resin build-up in the cyclone, some systems add resin to the fiber after steam separation. However, if resins that emit relatively high levels of VOCs are used, the resulting VOC emission levels may also be relatively high (i.e. beyond the acceptable MACT regulations). In addition, it has proven difficult to achieve adequate blending of the resin with the fiber material when resin is added downstream of the separator in non-pressurized systems. Such atmospheric systems often result in fiber clumping and/or resin spotting on the product, as noted above. Some pressurized systems may achieve sufficient blending, but require that a percentage of dirty steam from the refiner continue into the dryer with the fiber. Thus, efficiency and effectiveness are reduced.
Therefore, most current refining/drying systems add resin in the line from the refiner to the separator to achieve adequate blending, at the cost of resin build-up problems noted above.
Therefore, there is a need for a fiber refiner steam separation system that is efficient and relatively low cost. The system must also provide for excellent blending of the fiber/resin mixture, and substantially reduce VOC emissions, preferably by at least about 75%.